Short answers, pulled from the story.
Capcom designer Takashi Nishiyama created Ken while directing development of the original 1987 Street Fighter, drawing on ideas from the martial arts film Spartan X. Budget and scheduling limits meant Ken shared Ryu's moveset from the start.
Ken and Ryu share moves because Capcom's budget and scheduling problems reduced an originally larger planned cast down to just the two of them. That shared moveset later became the basis for their characterization as friendly rivals and fellow students of Gouken.
Ken Masters marries his girlfriend, Eliza, according to his ending in Street Fighter II. The couple's son, Mel, first appears as a child in Ken's Street Fighter III ending and grows into a teenager by Street Fighter 6.
In Street Fighter 6, Ken is framed by deepfake footage after the terrorist leader JP manipulates him into unknowingly funding a group called Amnesia. Ken goes into hiding to protect his family and is later found keeping a low profile at a construction site in the game's World Tour mode.
Violent Ken is an alternate, more powerful persona created for SNK vs. Capcom: SVC Chaos and Street Fighter II, resulting from the villain M. Bison manipulating Ken with his Psycho Power. Ken breaks free of the persona upon seeing a red bandana given to him by Ryu in Street Fighter Alpha 2, and Violent Ken later became a full playable character in the 2017 update Ultra Street Fighter II: The Final Challengers.
Reuben Langdon voiced Ken in Street Fighter IV, Super Smash Bros. Ultimate, and Street Fighter V, before David Matranga took over for Street Fighter 6. In live action, Damian Chapa played Ken in the 1994 Street Fighter movie, Christian Howard played him in the Street Fighter: Legacy series, and Noah Centineo is set to play him in an upcoming reboot.